ATAVISM. 159 



Among reptiles, birds, and mammals, our knowledge at 

 present only warrants us in regarding extra digits as 

 reversions when they occur in non-pentadactyle animals. 

 For instance, the spider monkeys have no thumbs ; a 

 careful dissection of the hand, however, will reveal, in 

 connection with the trapezium, a band of fibrous tissue 

 containing nodules of hyaline cartilage, representing the 

 missing thumb. On one occasion I was surprised to 

 find in an adult specimen of Ateles panniscus a small 

 thumb projecting above the web of the finger and 

 furnished with a nail. This condition is, I understand 

 from competent zoologists, not infrequent, and as it is 

 an example of the attainment of a functional condition 

 by an organ usually suppressed in this species, it comes 

 within the definition of atavism. Further, those mon- 

 keys, its closest allies, marmosets, Mycetes, and Callithrix, 

 possess functional thumbs. 



All cases of extra digits in non-pentadactyle mammals 

 are not necessarily atavistic : let us consider this in 

 reference to the horse. That the modern horse walks 

 upon the enlarged third digit, and has a vestigial meta- 

 carpal or metatarsal on each side of it, is accepted by 

 morphologists. The comparatively recent ancestors of 

 the horse had three functional digits. Hensel's investi- 

 gations on Hipparion mediterraneum indicate the pro- 

 bability that the inner (second) digit was the last to abort. 

 Horses are occasionally seen with two functional digits 

 instead of one, and it is a noteworthy circumstance that 

 in the majority of cases it is the inner digit which 

 reappears, that is, the one which we should theoretically 

 expect to reappear most frequently. Such cases are 



